Interactive computer displays on which a user draws with a writing implement that simulates a marking pen and/or a pointing device are particularly attractive for use in classrooms as replacements for white boards. Such systems typically include a display that provides an image on a large format computer display or a projected image on a vertical surface and a “pen” which the user uses to “write” on the display. The projected image typically includes material that the user wishes to discuss and highlight during the user's lecture. The pen has a tracking mechanism that allows the system to monitor the pen position and the status of one or more buttons on the pen. When the user “writes” on the display with the pen, the system tracks the position of the pen and updates the display to show a line at the locations traversed by the pen. The pen can also be used as a pointing device for activating various controls that are represented by “buttons” on the displayed image. The user activates the controls by pressing the pen to the button object on the displayed image. The user signals the system as to the mode of use by pressing one or more buttons on the pen.
One class of pen tracking systems utilizes a pen that emits ultrasound pulses together with infrared synchronizing pulses. The pulses are received by two or more sensors that are mounted on the display surface. The sensors are spaced apart from one another by a known distance. By measuring the difference in time of arrival between the infrared pulses and the ultrasound pulses the position of the pen relative to the sensors can be determined. This type of system has the advantage of tracking the pen even when the user is between the projector and the display surface.
The pen generates a modulated sine wave at an ultrasound frequency. The receivers and associated electronics must detect the beginning of the modulation envelope to determine the time of arrival at each receiver. The precision of the system depends on the assumption that the pen generates a particular modulated envelope that is constant in shape independent of the location of the pen on the writing surface.
The acoustical transmitter on the pen is typically mounted above the end of the pen so that the pulses originate from a location that is above the writing surface. Since the writing surface is typically a good sound reflector, a virtual image of the transmitter is generated in which the virtual transmitter appears to be on the other side of the writing surface. When the acoustical receivers are mounted on the writing surface, the arrival times of the signals from the real pen and the virtual pen are essentially the same, and there is no significant interference between the two signals.
However, there are important applications in which the receivers must be mounted above the writing surface; for example, interactive displays in which the receivers are incorporated in a projector mount requiring that the receiver be mounted above the writing surface. When the acoustical receivers are mounted above the writing surface with an offset more than a quarter of the wavelength, difference in arrival times between the real pen and virtual pen signals can be as high as half of the wavelength. In that case, interference between the wave forms can be significant and the distortions can cause the detection algorithm to fail. The worst case occurs when the distance between the real pen and virtual pen is an odd multiple of half wavelength.